Chapter 7: Vertebrates

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Chapter 7: Vertebrates

Vertebrates are one of the most conspicuous organisms that cause damages to the forest stands. Their body size varies from few centimeters and grams of small vertebrates like mice (Mus spp.) to withers height over than two meters and weight close to one ton in some bovines. Nevertheless, the body size does not need to be a correlate for amount of damages in forests.

Box 7.1. Extinct in the wild

Wisent (Bison bonasus) is a European bison, which extinct in the wild due to hunting in the beginning of 20th century. This is second known species of bison after well-known American bison (Bison bison). Several individuals of wisent were kept alive in captivity in Poland. Thank to this action, they were recently reintroduced to the wild in several places in Europe. Presently, it is the heaviest animal living in the wild on European continent. Its height could be close to two meters and weight close to one ton. Compared to its American relative, wisent is rather forest-dwelling animal and it is also indicated that this species browse more, and graze less than American bison.

Tobias Kuemmerle and colleagues found that wisent is highly dependent on mosaic landscape and that area of broad-leaved forests is the most important for species survival. This illustrates what are the most important places of future possible damages caused by wisent in Europe. The important fact from this point of view is that presence of these habitat types outside the protected areas would lead to the future conflict. Due to the possible large migrations among metapopulations of wisent, problems probably would be higher outside of east Europe. The tendency for migration was documented by Rafal Kowalczyk and colleagues, who find that the largest movements are out of the vegetation period – i.e. when the browsing in forests is almost higher. Philip Schmitz and colleagues documented the movements and habitat preference also to the plantation forests in west Europe. One of the examples of possible conflict out of the East Europe was that wisents preferred spruce forest. Atle Mysterud with colleagues found that the survival of this big mammal is also dependent on masting of oak. Thus, this large ruminant could be also a competitor for acorns. On the other hand, present extant of damages caused by this big animal appears to be very low and also of low commercial interest.

Kowalczyk R., Krasinska M., Kaminski T., Gorny M., Strus P., Hofman-Kaminska E., Krasinski Z.A. (2013) Movements of European bison (Bison bonasus) beyond the Bialowieza Forest (NE Poland): range expansion or partial migrations? Acta Theriologica 58: 391-401.

Kuemmerle T., Radeloff V.C., Perzanowski K., Kozlo P., Sipko T., Khoyetskyy P., Angelstam P. (2011) Predicting potential European bison habitat across its former range. Ecological Applications 21: 830-843.

Mysterud A., Barton K.A., Jedrzejewska B., Krasinski Z.A., Niedzialkowska M., Kamler J.F., Stenseth N.C. (2007) Population ecology and conservation of endangered megafauna: the case of European bison in Bialowieza Primeval Forest, Poland. Animal Conservation 10: 77-87.

Schmitz P., Caspers S., Warren P., Witte K. (2015) First Steps into the Wild–Exploration Behavior of European Bison after the First Reintroduction in Western Europe. PLoS ONE 10: e0143046.

The smallest vertebrates like voles (Microtus spp.) have highly fluctuating population densities and thus total body size of all individuals might outnumber the body size of large mammals. Also the damages caused by voles during the peak of their population densities might be higher than those caused by herd of wisents (Bison bonasus). Small mammals cause the greatest damages to the forest stands in the time when they go through the peak in population densities. They are the most harmful in afforested clear-cuts and on natural tree regeneration under the canopy of mature forest stands. They cause damages to the roots, because they severe seedlings. Moreover, they also damage bark of seedlings. This kind of damage is often caused also by hares (Lepus spp.) and rabbits (e.g. Oryctolagus cuniculus). Furthermore, small rodents like squirrels (Sciurus spp.) or dormouse (Gliridae) often forage on seeds of target tree species. This might cause worse natural regeneration in some years.

Beaver (Castor spp.) as rodent with relatively large body size and the second largest rodent on Earth is able to cut down mature trees. Even if, there appears to be some preference of beavers to softwood broadleaved trees like poplars and aspens (Populus spp.) together with willows (Salix spp.), beavers cause also damages on all other target tree species during the building of their dams and homes. Their damages might be relatively dangerous within the woodlands that are close to human settlements, because beavers are able to cause local floods or falls of trees on the roads with higher intensity of transport. Moreover, they can cause the fall of forest regeneration in floodplain areas. Forest protection against damage by beavers is considerably complicated in some countries, because Eurasian beaver (Castor fiber) is protected by legislation.

Several species with medium-large body size like roe deer (Capreolus capreolus) and wild boar (Sus scrofa) are mostly foraging within both forest and agricultural landscapes and, thus, their damages to the forest are sometimes season-dependent. This is, for example, the case of males of roe deer that fray on saplings during the time of marking of their territories. Even if they spent a lot of time outside of the forested areas, they can cause damages to the trees in open landscape – e.g. unsuccessful afforestation on agricultural land. Hogs often grout in upper soil and uprooted seedlings and feed on seeds during the winter time. Nevertheless, this activity might be sometimes useful, because of scarified of vegetation cover which is open for natural regeneration and also causes the lower densities of larvae of some insect pests.

Wild ungulates with large body like red deer (Cervus elaphus) cause nearly the same damages like other deer and moose (Cervidae), although they are more dependent on forests and woodlands. The largest damages are mostly at the end of vegetation season, when they search for minerals and strip the bark of mature trees. This causes damages to the commercially important part of the tree – i.e. lower part of the stem. During the winter time they often browse on seedlings and feed on buds. In this place it will be good to write, that not only ungulates cause damages to the forest trees, because e.g. bears (Ursus spp.) sometimes damage trees – well known are damages caused by claws of male bears on endemic Pinsapo fir (Abies pinsapo) in Spain.

Many approaches against damages of ungulates are presently used. Except of human regulation of their population densities by shooting and natural regulation caused by factors like predation or parasitism, there are several other relatively effective approaches. One of them is use of repellents, although it is indicated that especially wild boars often become accustomed to repellents after relatively short time-period. Relatively good way is alternation of different substances (e.g. smell and odor repellents).

Serious problems, especially during the afforestation or on natural regeneration are due to pasture of domestic animals. In this case is the most often cause the competition between agricultural and forest land. One of the possibilities of regulation of pasture is building of fences or creation of ditches. On the other hand, in same places on Earth, this approach is too expensive and thus the only one prevention against damages is in the hands of herdsmen.

During the time of protection against small rodents, poisonous traps were sometimes used. However, their negative impact on environment (e.g., soil and non-target animals) could be higher than commercial losses caused by small rodents and poisonous traps are prohibited in several countries. Other mechanical approaches are for example the use of fences around the clear cuts – unfortunately, they are not so much effective against wild boars, hares and rabbits – because hogs can damage the wire and small mammals could climb a mesh. Another option is individual tree protection, like tree guards – mostly plastic for broadleaves and wire for conifers. In this place, it would be good to write that there are many other possibilities how to protect target tree species against vertebrates, while some of which are country or site dependent or specific.

Figure 7.1. Silver fir (Abies alba), Scots pine (Pinus sylvestris) and European hornbeam (Carpinus betulus) naturally regenerated in the spruce bark beetle (Ips typographus) gap and under the canopy of Norway spruce (Picea abies) stand. Pine is damaged by fraying. Fir and hornbeam are browsed by roe deer (Capreolus capreolus) that reach high game stocks in rural agricultural-forest landscape of the Central Europe. Only regeneration of Norway spruce is left undamaged by high game stocks.


Example 7.1. Bratton S.P. (1975) The effect of the European wild boar, Sus scrofa, on gray beech forest in the Great Smoky Mountains. Ecology 56: 1356-1366.

Wild boar (Sus scrofa) is still increasing abundance in its former distribution area, in spite of that before no more than one century was close to extinction in the wild. This vertebrate was introduced during 1940s to the North America and colonizes many new places. The species surely has some impact on forests, however, its damages in forestry appears not to be as high as in agriculture. Moreover, this species might be a beneficial, because of its omnivorous diet, of which part are also larvae of insect pests.

Susan P. Bratton studied this species in Great Smoky Mountains National Park in U.S.A. Her main aim was to find possible impact of this exotic species on native ecosystem. She measured canopy and understory in both hog-occupied and hog-free areas. The activity of wild boars had no significant impact on canopy species. Hog rooting was highest in places with high moisture and decreased toward dry sites. Rooting activity reduced species richness, but not the diversity, which showed relatively high values in disturbed plots. This example illustrates that the impact of exotic species do not need to be always negative.

Example 7.2. Motta R. (1996) Impact of wild ungulates on forest regeneration and tree composition of mountain forests in the Western Italian Alps. Forest Ecology and Management 88: 93-98.

Forests in Europe, also in less accessible areas, are often damaged by high game stocks. The main types of damage are bark stripping on mature trees, which is lowering the outcomes from the timber, browsing on seedlings and saplings and fraying caused by antlers of males. Red deer (Cervus elaphus) and roe deer (Capreolus capreolus) are one of the most importanttree- damaging ungulates in many parts of Europe.

Renzo Motta studied the impact of ungulates on forests in Italy (Europe). He focused on bark stripping, browsing and fraying. He found that the damages on forest regeneration occurred from approximately 10 % to nearly 80 %. The most sensitive tree species was silver fir (Abies alba). Browsing and bark stripping was more selective than fraying, although there was a difference in fraying between red and roe deer. He also found that fraying caused high lethality under relatively low game stocks. Damages caused by browsing increases rapidly under high game stocks.

Example 7.3. Curtis P.D., Jensen P.G. (2004) Habitat features affecting beaver occupancy along roadsides in New York state. Journal of Wildlife Management 68: 278-287.

Beaver (Castor spp.) is known to be highly effective ecosystem engineer that is creating habitats for many other species. On the other hand, its activity could bring potential threat when closing to the human settlements and infrastructure. Beaver is able to cut trees of high dimensions and, thus, it is able to cause local floods or car-accidents on highly frequented roads.

Paul D. Curtis and Paul D. Jansen (2004) studied what habitat parameters are suitable for the presence of beaver. The study was done in U.S.A. The aim was to find implications for management suitable for presence of beaver and avoidance of damages to the roads. They found that with rising proportion of woody vegetation along roads there was also rising amount of damages caused by beaver. Thus, the main implication would be quite impractical – removal of trees along the roads, although this result might have some practical impact, especially, during planning of establishment of new roads.

Example 7.4. Huitu O., Kiljunen N., Korpimaki E., Koskela E., Mappes T., Pietiainen H., Henttonen H. (2009) Density-dependent vole damage in silviculture and associated economic losses at a nationwide scale. Forest Ecology and Management 258: 1219-1224.

Small mammals, as voles, are able to cause damages to large areas of forests. Their damages are mostly correlated with population densities within the cycles during the years. This means that in some years of lethal phases they are close to be rare, while in the time of the peak of their population densities, it is nearly impossible to prevent forests against vole damages. Their damages are also very important, because they occur worldwide. Voles mostly forage of on roots, bark and seeds of target forest tree species, they also severe tree seedlings.

Otso Huitu and colleagues studied impact of vole populations in the time, when population densities were highest for over fifteen years in Finland. They found that vole destroyed nearly five million tree seedlings with total cover of area of 2,600 hectares. Moreover, 80% of damages were on the commercially most important tree species – Norway spruce (Picea abies). Even if, it is hard to predict high population densities of voles in the future, they find that damages during the winter time were correlated with abundances in previous autumn. This might be an effective monitoring tool for damages caused by voles in forests.

Example 7.5. Mikich S.B., Liebsch D. (2014) Damage to forest plantations by tufted capuchins (Sapajus nigritus): Too many monkeys or not enough fruits? Forest Ecology and Management 314: 9-16.

Ongoing loss of natural habitats leads to the higher damages on plantations by wild animals. Majority of damages caused by vertebrates in forests that are reported are caused by ungulates or voles. Primates are known to sometimes damage the crop in agriculture. Sapajus nigritus, is a capuchin monkey distributed in the Atlantic Forest in South America. This species is considered to decrease its population densities and total distribution area due to the habitat loss. Recently, there have been reports that this monkey is causing damage to commercial pine plantations in Brazil by bark-stripping.

Sandra Bos Mikich and Dieter Liebsch studied this monkey in Parana state (southern Brazil). They found that population densities of this monkey were lower than was predicted – mean number of individuals was not higher than three monkeys per square kilometer. They also found that the damages were the highest during the time of low fruit and seed availability within the year. The option, how to decrease the damages caused by the monkeys is, thus, the use of less attractive trees. Planting of some trees that provide seeds and fruits during the period of starving within the plantations should lead to the lower damages from the longer time perspective. Supplementary feeding of game during the winter is well known and almost effective against the damages in Europe.

Example 7.6. Sullivan T.P., Nordstrom L.O., Sullivan D.S. (1985) Use of predator odors as repellents to reduce feeding damage by herbivores. Journal of Chemical Ecology 11: 903-919.

Repellents are one of the methods used against the damages of animals. Their possibility of use is quite large and except of use in agriculture, repellents are commonly used to ensure the highways against the vehicle collisions with animals. They are also used against the damages caused by wild animals in forests. The snowshoe hare (Lepus americanus) is a North American species of hare. During the winter time, it forages on twigs, tree bark and buds and might cause damages to the plantation forests.

Thomas P. Sullivan and colleagues tested the odors of wild predators as repellents against the snowshoe hare. They tested success of hare consumption of willow browse and coniferous seedlings in British Columbia (U.S.A.). They found that lynx and bobcat feces, weasel anal gland secretion, together with lynx, bobcat, wolf, coyote, fox, and wolverine urines resulted in the most effective suppression of hare feeding damage. Lodgepole pine (Pinus contorta) seedlings were the best protected by weasel scent. These results were one of the first that indicated the advances in use of predator odor toward the future. One problem reported was quite expected short-term effectiveness of these treatments. They worked up to seven days, which was mainly due to evaporative loss of the active repellent components.

Example 7.7. Klich D. (2017) Selective bark stripping of various tree species by Polish horses in relation to bark detachability. Forest Ecology and Management 384: 65-71.

The Konik is a polish primitive horse, it is indicated that this horse is one of the possible descendants of wild horses that were common in the past in Europe. This species is quite small horse, which weight not exceeds 400 kilograms. It was used as a transport animal, but recently it is mainly kept in the wild or semi-wild conditions in nature reserves. Presently this horse is a subject of new grazing management in European nature reserves.

Daniel Klitch studied the Konik population in one acclimatization enclosure in Poland. He was mainly interested in relationship between the bark stripping intensity caused by these Polish primitive horses and the degree of bark detachability. The focus was on diameter in the breast height and the tree species. Only nine of twenty found tree species were damaged. Goat willow (Salix caprea) was the most damaged, although it was not the dominant tree species. Eight of ten often damaged tree species had decreasing damages with increasing diameter. Thus, the relative force to detach the bark is probably the most important factor for selection (i.e. bark stripping) of particular tree species by the horse. These wild horses, thus, preferred admixed tree species of lower diameter. One of the possibilities against the damages caused by such animals appears to be individual protection of trees using tree guards.

Example 7.8. Kowalczyk R., Taberlet P., Coissac E., Valentini A., Miquel C., Kaminski T., Wojcik J.M. (2011) Influence of management practices on large herbivore diet – case of European bison in Bialowieza Primeval Forest (Poland). Forest Ecology and Management 261: 821-828.

Increasing human population, hunting or change in land use caused lowering or even extinction of several wild animals. One of the possible reasons in the mosaic of vanishing of some wild animals from landscape in Northern hemisphere appears to be the competition for natural sources, which is known especially for predators. Another reason could be damages on crop fields or plantation forests. Large wild ungulates often cause (or have big potential) damages to the forest trees.

Rafal Kowalczyk with colleagues studied the wisent (Bison bonasus) in its new home of Bialowieza forest in Poland. This big animal is able to change its environment and due to its quite strong relationship with forests, it might cause damages to the target tree species in the future. Authors were interested in the diet structure in differently managed populations – differences were in access to supplementary fodder. They found that wisent groups differed significantly in their diet. Herbs were almost dominant part of their diet. Woody material in their faecal samples increased with decreasing access to supplementary fodder. On the other hand, tree species that were browsed by wisent were mainly of low economic importance for forestry (e.g. hornbeam, birch or willow). The results indicated the need for supplementary feeding of wild ungulates in plantation forests. Nevertheless, in the case of wisent the feeding should be applied carefully due to negative impact on health of wisent populations.

Figure 7.2. Several forest tree species like European ash (Fraxinus excelsior) are able to outgrow the damages caused by browsing of wild ungulates. The main problem is that the most commercially important part of the stem (i.e. lower part) is deformed and these individuals will be cut before mature age or the yield in mature age will be significantly lower.


Question 7.1. Larvae of some insect pests overwinter in the forest soil. It is very hard to kill them using any known mechanical or chemical approach. However, there are several natural enemies (e.g. fungi or bacteria) that are able to decrease the numbers of larvae in the soil. What is one of the most efficient medium-large vertebrates that cause reduction of the number of pest larvae in the soil?

Question 7.2. Large ungulates cause many types of damages in forests. These damages are mostly seasonally dependent. This means that, for example, seedlings are mostly not browsed during the summer months, when game has diversified supply of food. What is the name for damages caused by antlers of males of deer?

Question 7.3. Beaver (Castor spp.) is the second largest rodent in the world. This animal is highly dependent on aquatic environment and it needs woody plants for building of its dams, canals, and lodges (homes). What kind of damages to the forest is caused by beaver?

Question 7.4. Population densities of voles (Microtus spp.) are going through cycles. When these small rodents reach high abundance, they become serious pests for forest stands and forest nurseries. Which stage trees are the most damaged by small mammals like voles?

Question 7.5. Wild ungulates often cause the damages to the target trees during the time when food supply is scarce. What is one of the most effective measures to ensure trees against the damages?

Question 7.6. Repellents are one of the methods used against the damages of animals in the forests. Which odors can be used as repellents against damages cause by small mammals?

Question 7.7. Wild ungulates often damage the stems of target tress individually distributed within the forest stands. What is one of the possible measures, how to protect individual saplings of target tree species?

Question 7.8. Monkey are not often studied as animals that have potential to became a pest in forests. What is the most important damage caused by primates to the forest trees?

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